I try to use the yeast within a month. I have, on occassion, dumped it into a flask and stired it with some fresh DME and water to get it going, but only rarely.

Just two weeks ago I used a baggie of 1056 that was from July (Late October when used, so 14-15 weeks, and it did fine. Slower than usual signs of activity (a little at 36 hours, good robust krausen at 4 days). However, I have also thrown out baggies as they got older and just said hell with it, I am going to get some fresh.